A pulsed gas-discharge laser usually includes a sealed enclosure filled with a gas mixture (laser gas). A series of electric discharges is struck in the lasing gas in a discharge region between spaced-apart electrodes. This is accomplished by applying a repetitively pulsed electrical potential across the electrodes. A laser resonator is arranged with an axis thereof extending through discharge region. The discharge energizes the gas mixture and the energized gas mixture provides optical gain. Laser output is delivered from the resonator in a series of optical-radiation pulses having a repetition frequency corresponding to the repetition pulsed electrical potential.
A pulsed gas-discharge laser commonly used in industrial applications is a pulsed carbon-dioxide (CO2) laser commonly referred to as a slab laser. In such a laser the spaced apart electrodes are elongated electrodes (“slab” electrodes), usually having a plane face of one arranged face-to-face and parallel to a corresponding plane face of the other. In such a CO2 laser, the lasing gas pressure is usually between about 50 Torr and 150 Torr. The pulsed electrical potential is applied as a pulsed radio frequency (RF) potential. The RF potential (power) during each pulse ignites and sustains the gas discharge. It is usual to provide a pre-ionizing device to create ionization in the lasing gas before the pulsed RF-power is applied.
In the absence of such a pre-ionizing device, the time required to ignite the discharge between the slab electrodes and obtain pulsed laser output can vary randomly. Such a random ignition time would be undesirable for applications requiring precise laser turn-on and turn-off time, such as in drilling, marking, engraving, scribing, and cutting. In addition, in order to ignite the discharge without a pre-ionizer, it would usually be necessary to increase the RF power to a level two or more times greater than the power necessary to sustain the discharge once it has been ignited. This adds complexity and cost to the RF power supply.
One prior-art approach to providing pre-ionization in a pulsed CO2 laser is described in U.S. Pat. No. 5,434,881. In this approach, the pre-ionization is provided by repeatedly striking a spark discharge between two auxiliary spaced-apart electrodes located in the vicinity of the discharge region. It has been found, however, that these auxiliary electrodes are rapidly eroded by the repetitive sparking, and that the eroded (sputtered) material of the electrodes can contaminate the lasing gas and shorten the lifetime of the laser.
One device designed to overcome the sputtering and contamination problems of the approach of the '881 patent is described in U.S. Pat. No. 6,963,596, to Shackleton et al., assigned to the assignee of the present invention and incorporated herein by reference. In this device, a pre-ionizing discharge is formed between two pin-like electrodes (pin-electrodes), each thereof covered by a dielectric jacket. The dielectric jacket for the pin electrodes is provided by a ceramic crucible having hollow extension portions protruding from a base of the crucible, and shaped to accommodate the pin-electrodes. The crucible is clamped into an aperture of the lasing gas enclosure, and a separate assembly including the pin-electrodes is clamped to the crucible. The dielectric-covered pin-electrodes are energized by a low-power RF power source.
The dielectric covering of the pin-electrodes of Shackleton et al. device essentially eliminates problems of sputtering and related contamination of the laser. However, parts for the device have been found to be difficult to fabricate, intricate to assemble and relatively fragile. There is a need for a simpler, more robust device that is equally effective at eliminating sputtering and contamination problems of prior art pre-ionization approaches.